Amphibians are ecologically important in most freshwater and terrestrial habitats in the United States: they can be numerous, function as both predators and prey, and constitute great biomass. Amphibians have certain physiological (e.g., permeable skin) and ecological (e.g., complex life cycle) traits that could justify their use as bioindicators of environmental health. For example, local declines in adult amphibians may indicate losses of nearby wetlands. The aquatic breeding habits of many terrestrial species result in direct exposure of egg, larval, and adult stages to toxic pesticides, herbicides, acidification, and other human-induced stresses in both aquatic and terrestrial habitats. Reported declines of amphibian populations globally have drawn considerable attention (Bury et al. 1980; Bishop and Petit 1992; Richards et al. 1993; Blaustein 1994; Pechmann and Wilbur 1994).

Approximately 230 species of amphibians, including about 140 salamanders and 90 anurans (frogs and toads) occur in the continental United States. Because of their functional importance in most ecosystems, declines of amphibians are of considerable conservation interest. If these declines are real, the number of listed or candidate species at federal, state, and local levels could increase significantly. Unfortunately, because much of the existing information on status and trends of amphibians is anecdotal, coordinated monitoring programs are greatly needed.

Faunal Comparisons
North American amphibian species exhibit two major distributional patterns, endemic and
widespread. Endemic species (Figure) tend to have small ranges or are restricted to specific habitats (e.g., species that occur only in one cave or in rock talus on a single mountainside). Declines are documented best for endemic species, partly because their smaller ranges make monitoring easier. Populations of endemics are most susceptible to loss or depletions because of localized activities (Bury et al. 1980; Dodd 1991). Examples of endemic species affected by different local impacts include the Santa Cruz long-toed salamander (Ambystoma macrodactylum croceum) in California, the Texas blind salamander (Typhlomolge rathbuni) in Texas. and the Red Hills salamander (Phaeognathus hubrichti) in Alabama; these three species are listed as federally threatened or endangered.

The number of endemic species that have suffered losses or are suspected of having severe threats to their continued existence has increased in the last 15 years (Table). In part, the increase reflects descriptions of new species with restricted ranges, but the accelerating pace of habitat alteration is the primary threat.
The ranges of most endemics in the western states (26 species) are widely dispersed across the landscape. In contrast, endemics in the eastern and southeastern states (25 species) tend to be clustered in centers of endemism, such as in the Edwards Plateau (Texas), Interior (Ozark) Highlands (Arkansas, Oklahoma), Atlantic Coastal Plain (Texas to Virginia), and uplands or mountaintops in the Appalachian Mountains (West Virginia to Georgia).

Widespread species often are habitat generalists. Many were previously common, but have shown regional or rangewide declines (Hine et al. 1981; Corn and Fogelman 1984; Hayes and Jennings 1986; Table). Reported declines of widespread species often lack explanation, perhaps because these observations have only recently received general attention or because temporal and spatial variations in population sizes of many amphibians are not well understood. Some reports are for amphibians in relatively pristine habitats where human impacts are not apparent.

A few examples of declines in widespread species illustrate the threats they face across the country:

* Amphibians predominate in small forest streams of the Pacific Northwest. Because timber is harvested without adequate streamside protection, many populations of the tailed frog (Ascaphus truei) and torrent salamanders (Rhyacotriton spp.) have been severely affected; some populations soon will warrant consideration for listing.
*The western toad (Bufo boreas) once was common in the Rocky Mountains, but now occurs at fewer than 20% of known localities from southern Wyoming to northern New Mexico.
* Many salamander and frog populations in the southeastern United States have been negatively affected, some severely, because of degradation of stream habitats (e.g., the hellbender, Cryptobranchus alleganiensis) and conversion of natural pinewood and hardwood forests and associated wetlands (e.g., gopher frog, Rana capito) to plantation forestry, agriculture, and urban uses.
* Leopard frogs (Rana spp.) which are used in teaching and research institutions, were once abundant in most of the United States. Populations in this diverse group have declined, sometimes significantly, in midwestern, Rocky Mountain, and southwestern states.

Causes of Declines
No single factor has been identified as the cause of amphibian declines, and many unexplained declines likely result from multiple causes. Human-caused factors may intensify natural factors (Blaustein et al. 1994b) and produce declines from which local populations cannot recover and thus they go extinct. Known or suspected factors in those declines include destruction and loss of wetlands (Bury et al. 1980); habitat alteration, such as impacts from timber harvest and forest management (Corn and Bury 1989; Dodd 1991; Petranka et al. 1993); introduction of non-native predators, such as sportfish and bullfrogs, especially in western states (Hayes and Jennings 1986; Bradford 1989); increased variety and use of pesticides and herbicides (Hine et al. 1981); effects of acid precipitation, especially in eastern North America and Europe (Freda 1986; Beebee et al. 1990; Dunson et al. 1992); increased ultraviolet radiation reaching the ground (Blaustein et al. 1994a); and diseases resulting from decreased immune system function (Bradford 1991; Carey 1993; Pounds and Crump 1994).

Amphibian populations also may vary in size because of natural factors, particularly extremes in the weather (Bradford 1983; Corn and Fogelman 1984). The size of amphibian populations may vary, sometimes dramatically, from year to year, so what is perceived as a decline may be part of long-term fluctuations (Pechmann et al. 1991). The effect of global climate change on amphibians is speculative, but it has the potential for causing the loss of many species.

Monitoring Needs
A profound need exists for national coordination of regional inventories and population studies, including a national effort to monitor amphibians on parks, forests. wildlife refuges, and other public lands. Only through long-term studies will better data on population changes through time and between sites become available. Such data are essential to evaluating the status and trends of amphibian species in the United States. Some regional surveys and inventories exist but only for a few species; these studies should be expanded into a coordinated effort with long-term monitoring of populations at many sites across the country as the goal.

In addition, more research is needed to determine the impact of natural and human-caused factors on the different life-history stages and environments of amphibians. Also, the assumption that amphibians are good indicators needs to be tested rigorously (Pechmann and Wilbur 1994). Likewise, understanding the dynamics of populations between habitats and regions, and the roles amphibians play in aquatic and terrestrial ecosystems is essential. Detailed work on the ecology of species and the factors implicated in declines needs to continue.